Advanced power electronic designs, especially those of aircraft generating systems, are currently challenged to meet more stringent design requirements, lower cost constraints and higher quality standards. To achieve these objectives, printed wiring boards are taking advantage of the smaller component size offered by surface mount technology components. However, some components are not yet available in an appropriate surface mount technology format and must be obtained as plated through-hole components. Designers are forced, therefore, to utilize components of mixed technology in the design of printed wiring boards.
The manufacturing of such mixed technology printed wiring boards containing both surface mount technology components and plated through-hole components typically involve a reflow soldering process followed by a wave soldering process. However, because of increased design performance requirements and more dense packaging of components, surface mount technology components may appear on both sides of a printed wiring board. This poses a problem because a reflow soldering process is not compatible with a wave soldering process when surface mount technology components appear on both sides of a printed wiring board. This non-compatibility necessitates a manual placement of the plated through-hole components, which is an extremely laborious task, magnifying the cost associated with manufacturing these mixed technology units.
The objective of providing surface mount technology components to both sides of a printed wiring board is addressed in a patent to Leibman et al., U.S. Pat. No. 5,167,361, which discloses a method of forming solder bumps on contact areas of both sides of the printed wiring board in a predetermined pattern. The solder paste is then reflowed and the solder joints on the first side are flattened. Tack media is then dispensed onto the joint areas for the subsequent placement of surface mount technology components on one side of the printed wiring board. A conventional reflowing process is again utilized to mount these components on the one side of the board. Subsequently, a second side of the printed wiring board is treated in the same manner as the one side of the board before surface mount technology components are mounted, after which the board is reflowed again. While the patent to Leibman et al. addresses mounting of surface mount technology components to a printed wiring board, it does not, as the invention to be described more fully hereinafter, avoid the binding between fine-pitched surface mount technology components, which results in short circuiting of the printed wiring board. Furthermore, the present disclosure eliminates the repeated reflow step before attaching surface mount technology components.
The problem of mounting surface mount technology components is also addressed in a patent to Johary, U.S. Pat. No. 4,722,470. The Johary patent teaches a method for applying discrete bodies of solder of predetermined size to leads of a component for subsequent surface mounting to a substrate by employing a solder transfer member. The transfer member is a plate having a non-wetted surface with an array of cavities matching the component lead pattern. The method includes placing solder paste on the transfer member and filling the cavities by wiping the plate surface. The component is then placed on the transfer member with the leads contacting the solder paste in the cavities. Reflow of the solder paste bonds to each lead a discrete body of solder having a precisely determined size. While the Johary patent addresses the problem of bridging of solder and the consequent short circuiting between adjacent solder joints, it is not directed, as the present invention, to combining both surface mount technology components and plated through-hole components on the same board. In addition, the method disclosed in the Johary patent incorporates directly placing solder on the leads of a component member rather than, as the invention described more fully hereinafter, directly applying a predetermined amount of solder to a printed wiring board.
Casting solder preforms into an array such that a plurality of preforms are initially formed as a single unit is disclosed in the patent to Lauterbach et al., U.S. Pat. No. 5,029,748. The solder preforms are either cast onto pins of a plated through-hole component or cast onto a separate carrier for subsequent insertion onto the pins of a plated through-hole component. While the Lauterbach et al. patent addresses the incompatibility of manufacturing processes associated with hybrid printed wiring boards, it does not, as the invention to be described more fully hereinafter, address the problem of sharing solder between adjacent joints of a component through the inconsistent manual placement of solder preforms. Such manual placement of solder preforms into an array takes away the precise control needed for fine pitched surface mount technology components.
The present invention is directed to overcoming one or more of the above problems.